In optical illumination systems, it is desirable to couple light from an illumination source (typically a lamp) into a light guide, or a plurality of light guides, as efficiently as possible. As the coupling efficiency improves, manufacturing and operating costs decline. When evaluating the coupling efficiency from any illumination source into any particular light guide, the designer must consider the size (and aspect ratio) and numerical aperture of both the source and the light guide. For optimum coupling, the image of the lamp on the light guide surface should match as closely as possible the spatial geometry and numerical aperture of the light guide.
High intensity white light sources used in illumination applications are typically extended sources (as opposed to point sources), because they have a finite illuminating element. The emitting surface cross-section is much greater than the cross-section of typical light guides. In addition, many lamps may be considered Lambertian or Quasi-Lambertian radiators. For such lamps, the radiation profile (far-field pattern) far exceeds the numerical aperture of the light guide. Since the numerical aperture and core cross-section of the light guide are both substantially smaller than the corresponding elements of the illumination source, it is impossible to efficiently couple the light into a single light guide. Consequently, a plurality of light guides must be placed at the output of the optical system in order to collect the light that exceeds the core cross-section and numerical aperture of a single light guide.
In addition to the near-field and far-field mismatch conditions between a lamp and a light guide, further geometric issues complicate source to light guide coupling efficiency. Since these illumination sources exhibit large aspect ratios, typically between 5:1 and 20: 1, then the near-field conditions of the lamp and the light guide are not easily matched (since light guides generally have an aspect ratio of one). If the entire image of the lamp is focused on the light guide end, then the light guide is greatly underfilled (spatially) in one dimension. If the image size is increased to match the minor axis of the image to the diameter of the light guide, ten the major axis of the image will greatly overfill the light guide. In either case, the coupling efficiency is not optimized.
Current multi-light guide couplers are designed to divide the radiation profile of extended illumination sources by placing a plurality of lenses around the source. Each lens collects only the solid angle of light emanated from the illumination source which can be accepted by the light guide; i.e. a solid angle less than or equal to the numerical aperture of the light guide.
A conventional way to launch light into a light guide is to focus the light onto the light guide end using a lens. This generally involves imaging the entire source as seen from the perspective of the lens. This technique can be very effective if the aspect ratio of the source is a close match to the aspect ratio of the light guide, or if the source is sufficiently small relative to the size of the light guide. However, if there is a large difference in aspect ratio, such as is the case for a high-pressure sodium lamp, which is long and narrow, then this brings about a loss of efficiency due to the mismatched geometry.